1. Field of the Invention
This invention relates to a process for an integrated circuit structure having a layer of low k carbon-doped silicon oxide dielectric material. More particularly, this invention relates to the removal of a photoresist mask used to form an opening such as a via or a trench in the low k dielectric material and the removal of residues from both the etching of the opening and the subsequent resist mask removal.
2. Description of the Related Art
In the continuing reduction of scale in integrated circuit structures, both the width of metal interconnects or lines and the horizontal spacing between such metal lines on any particular level of such interconnects have become smaller and smaller. As a result, horizontal capacitance has increased between such conductive elements. This increase in capacitance, together with the vertical capacitance which exists between metal lines on different layers, results in loss of speed and increased cross-talk. As a result, reduction of such capacitance, particularly horizontal capacitance, has received much attention. One proposed approach to solving this problem of high capacitance is to replace the conventional silicon oxide (SiO2) dielectric material, having a dielectric constant (k) of about 4.0, with another dielectric material having a lower dielectric constant to thereby lower the capacitance.
In an article by L. Peters, entitled “Pursuing the Perfect Low-K Dielectric”, published in Semiconductor International, Volume 21, No. 10, September 1998, at pages 64–74, a number of such alternate dielectric materials are disclosed and discussed. Included in these dielectric materials is a description of a low k dielectric material having a dielectric constant of about 3.0 formed using a chemical vapor deposition (CVD) process developed by Trikon Technologies of Newport, Gwent, U.K. The Trikon process is said to react methyl silane (CH3—SiH3) with hydrogen peroxide (H2O2) to form monosilicic acid which condenses on a cool wafer and is converted into an amorphous methyl-doped silicon oxide which is annealed at 400° C. to remove moisture.
An article by S. McClatchie et al. entitled “Low Dielectric Constant Oxide Films Deposited Using CVD Techniques”, published in the 1998 Proceedings of the Fourth International Dielectrics For ULSI Multilevel Interconnection Conference (Dumic) held on Feb. 16–17, 1998 at Santa Clara, Calif., at pages 311–318, also describes the formation of methyl-doped silicon oxide by the low-k Flowfill process of reacting methyl silane with H2O2 to achieve a dielectric constant of ˜2.9.
The use of low k carbon-doped silicon oxide dielectric material formed by reacting methyl silane with hydrogen peroxide (the Trikon process) has been found to be capable of better gap filling characteristics than other low k materials. Good gap filling characteristics, in turn, can result in the formation of void-free filling of the high aspect ratio space between parallel closely spaced apart metal lines with dielectric material having a lower dielectric constant than that of convention silicon oxide, thereby resulting in a substantial lowering of the horizontal capacitance between such adjacent metal lines on the same metal wiring level.
However, it has been found that the bond formed between the silicon atoms and the organic groups in a carbon-doped silicon oxide dielectric material is not as stable as the silicon-oxygen bond found in conventional silicon oxide (SiO2) materials. For example, unprotected surfaces of such a low k carbon-doped silicon oxide dielectric material may be exposed to oxidizing or “ashing” systems, which are conventionally used to remove a photoresist mask from the layer of low k carbon-doped silicon oxide dielectric material, after formation of openings, such as vias, therein. It has been found that the ashing process results in damage to the bonds (cleavage) between the organic groups and the silicon atoms adjacent the surfaces of the layer of low k carbon-doped silicon oxide dielectric material exposed to such an ashing treatment. This cleavage of the carbon-silicon bonds, in turn, results in removal of such organic materials formerly bonded to the silicon atoms, along with removal of the organic photoresist materials from the integrated circuit structure. The silicon atoms from which the organic groups have been cleaved, and which are left in the damaged surface of low k carbon-doped silicon oxide dielectric material, are in a highly reactive state and become water absorption sites if and when the damaged surface is exposed to moisture.
This absorption of moisture by the damaged low k carbon-doped silicon oxide dielectric material, can result in hydroxyl bonding to the reactive silicon atoms left from the cleavage of the carbon-silicon bonds in the damaged surfaces of the low k carbon-doped silicon oxide dielectric material. This silicon-hydroxyl bond is not a stable bond, and subsequent exposure to heat, e.g., during subsequent processing such as annealing, can result in cleavage of the silicon-hydroxyl bond, thereby causing water vapor formation which, for example, can interfere with subsequent filling of a via/contact opening or a damascene trench with metal filler material, resulting in what is known as via poisoning.
The upper surface of the low k carbon-doped silicon oxide dielectric material can be protected from such attack during removal of the resist mask by provision of a protective layer, e.g. a capping layer of conventional silicon oxide (dielectric constant k of ˜4) over the upper surface. However, the use of the conventional ashing (oxidation) process to remove the resist mask causes physical damage to any carbon-doped silicon oxide material which is exposed in walls of vias, trenches, or contact openings, resulting in cracked, degraded, bowed, and porous insulating material in the walls of such openings. The pores in the walls of vias, trenches, or contact openings can present further problems by retaining destructive gases produced during one or more subsequent metal deposition steps. The physical damage to the insulating material which forms the walls of such openings cause the subsequent metal deposition step to be unreliable; and the presence, in the pore cavities, of gases produced during metal deposition steps result in a degradation of the metal/metal nitride properties.
Sukharev et al. U.S. Pat. No. 6,114,259, assigned to the assignee of this invention, and the subject matter of which is hereby incorporated by reference, teaches removal of the photoresist mask used to form openings such as vias in low k carbon-doped silicon oxide dielectric material in a two step process wherein the etched via sidewalls of the low k carbon-doped silicon oxide dielectric material are first treated with a nitrogen plasma, or a nitrogen and oxygen plasma, to densify the exposed low k carbon-doped silicon oxide dielectric material. The photoresist mask is then removed with a mild oxidizing agent comprising an H2O plasma. The H2O plasma removes the resist mask without damaging the exposed low k carbon-doped silicon oxide dielectric material comprising the sidewalls of the etched via sufficiently to interfere with later filling of the via with an electrically conductive metal filler.
Wang et al. U.S. Pat. No. 6,028,015, also assigned to the assignee of this invention, and the subject matter of which is also hereby incorporated by reference, teaches treating damaged via sidewalls of low k carbon-doped silicon oxide dielectric material with either a hydrogen plasma or a nitrogen plasma to repair the via sidewall surfaces which have been damaged by prior removal of the photoresist mask with a traditional ashing/oxidation process, i.e., an oxygen plasma. Such a treatment with a hydrogen or nitrogen plasma is said to cause the hydrogen or nitrogen to bond to silicon atoms with dangling bonds left in the damaged surface of the low dielectric constant carbon-doped silicon oxide insulation layer to replace organo material severed from such silicon atoms at the damaged surface. Absorption of moisture in the damaged surface of the layer of low dielectric constant carbon-doped silicon oxide dielectric material, by bonding of such silicon with moisture, is thereby inhibited.
Previously cited U.S. Pat. No. 6,316,354, discloses a process for removing resist mask material from a protective barrier layer formed over a layer of low k carbon-doped silicon oxide dielectric material of an integrated circuit structure without damaging the low k dielectric material, and without the necessity of subjecting the exposed via sidewalls of the low k dielectric material to either a pretreatment to inhibit subsequent damage to the low k dielectric material during the resist removal, or a post treatment to repair damage to the low k material after the resist removal. The resist removal process comprises exposing the resist mask material to a hydrogen plasma formed from a source of hydrogen such as ammonia, while maintaining the temperature below about 40° C. to inhibit attack of the low k silicon oxide dielectric material by oxygen released from the decomposition of the resist material.
Previously cited U.S. Pat. No. 6,562,700 describes a process for removing a photoresist mask used to form openings in an underlying layer of low k carbon-doped silicon oxide dielectric material of an integrated circuit structure formed on a semiconductor substrate, by exposing the photoresist mask in a plasma reactor to a plasma formed using a reducing gas until the photoresist mask is removed. In a preferred embodiment the reducing gas is selected from the group consisting of NH3, H2, and a mixture of NH3 and H2. The low k carbon-doped silicon oxide dielectric material is then treated with a solvent capable of dissolving etch residues left from forming the openings in the low k dielectric material, and from removing the photoresist mask used to form the openings in the low k carbon-doped silicon oxide dielectric material. The low k carbon-doped silicon oxide dielectric material is then annealed in an annealing chamber at a temperature sufficient to remove liquid and gaseous byproducts from the low k carbon-doped silicon oxide dielectric material.
While the substitution of a milder oxidation process using an H2O plasma, as proposed by Sukharev et al. U.S. Pat. No. 6,114,259, or the substitution of the reducing processes respectively proposed by Hu U.S. Pat. No. 6,316,354 and Gu et al. U.S. Pat. No. 6,562,700, constitute improvements over the pg,7 conventional oxidation or ashing process, it has been found that the problem of via poisoning still is experienced at a higher than desirable rate.
Thus, in the processing of low k carbon-doped silicon oxide dielectric material in integrated circuit structures, it remains a goal to provide for the removal of residues from both the via etch step, and removal of the photoresist via mask, in a manner which will not interfere with subsequent filing, with electrically conductive material such as metals, of the vias or other openings in the low k carbon-doped silicon oxide material.